On 7/2/2014 8:20 AM, Ray, W4BYG wrote:
Additionally, I would connect the analyzer with a half wave length of coax
instead of the 4 foot piece. The half wave length will reflect what the
actual impedance at the antenna feed is. The 4 foot piece will only do that
if/when the antenna impedance is actually at 50 ohms. Remember to calculate
the half wave length by the velocity factor of the cable you are using.
NEVER rely on a calculation for the electrical length of coax. Vf varies
with frequency, and there is also variation as a result of manufacturing
tolerances. Start with a piece of line that is at least 10% longer than
the computed length, connect it to your analyzer with a short at the far
end, and look for the frequency where you measure a very small R and
zero X. Now, trim the coax until that zero point is at the desired
frequency.
You could, of course, make this measurement with an open circuit at the
far end and look for a maximum value of R and zero X, but depending on
the analyzer, the short may be more sensitive. You could also look for a
short at half the operating frequency, but this would be subject to the
variation in Vf with frequency.
Vf of any coax starts out much lower than the published value at audio
frequencies, then rises to a value equal to the published value around
50 MHz. In the range of 160M - 80M, the value is typically 1-2% less
than the published value. For more on this, see
http://k9yc.com/Coax-Stubs.pdf
If you have an network analyzer or complex impedance analyzer that does
swept Z measurements and can write a Touchstone file, you can export
that file to Zplots and let Zplots compute and plot Zo and Vf vs
frequency for your cable. Zplots is an Excel spreadsheet developed by
AC6LA. It's a free download. See ac6la.com.
73, Jim K9YC
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